I am curious why you are assuming that a lighter trainset would allow for a narrower bore? The
ECLRT used 6.5m diameter TBMs while the
TYSSE used only 6.0m diameter TBMs. In fact, I believe the OL will have a larger diameter tunnel bore than that which was planned for the RLS/RLN.
The ECLRT primarily has a larger bore size than the standard TTC subway due to the use of an overhead caternary instead of a third rail, and it is entirely possible that the entirely grade-separated OL will be able to use a third rail instead of overhead wires. This doesn't necessarily mean the tunnel will be a narrower diameter than was planned for the RLS/RLN or than what is currently used for the TTC standard. A lot more goes into the design of the tunnel (including diameter) than just the train size. You have to consider airflow and ventilation, allowable clearance (especially on OL's particularly curvy track), safe passage for workers/emergency exit access, among other things.
Furthermore, having a smaller tunnel diameter (say, 6.0m vs 6.2m) doesn't have that much of a benefit to a project. Sure, the TBM is smaller and you'll excavate less soil/rock along the tunnelled alignment. This does very little for the required strength of the pre-case tunnel liners, the general size of the station (shaving 20cm off is
nothing here), ease of TBM operation, tunnelling efficiency, or overall cost of tunnelling.
Sure, if you're pipe-jacking 400m from one sewer access to another, going from 1.2m bore diameter to 1.0m bore diameter can save you 36% on material excavated. But, for example if we're dropping an approximately 18km (9km twin bore) tunnelled section from 6.2m to 6.0m, you're going to save what... 6-7%? Basically just on material disposal, likely not even on the overall TBM cost. The relatively small amount of material disposal cost saved is not something that is really critical in large scale transit projects such as this.